This invention relates to video signal reproducing systems, such as a VTR playback device, having at least two different reproducing heads and, more particularly, to apparatus for use with such a system wherein a reference synchronizing signal is phase-synchronized with the video signals which are reproduced by either head.
When a video tape recorder (VTR) is used in a reproducing mode, the video signals which are reproduced thereby may be supplied to a time base corrector to correct time base errors which may appear in the reproduced video signals. These time base errors, which are frequency and/or phase errors, may be attributed to expansion or contraction in the record medium after the video signals have been recorded thereon, or a variation in the speed at which the record medium is transported during the playback mode relative to the recording mode, or a change in the speed at which the playback heads scan the record medium. In a typical time base corrector, the reproduced video signals are written into a memory device at a rate which is synchronized with the time base errors, and these stored video signals subsequently are read out of the memory device at a relatively fixed, constant rate, thus eliminating such time base errors. Typically, the operation of the time base corrector, such as the starting time at which, for example, the first line of stored video signals is read out therefrom, is synchronized with the operation of the VTR. That is, a reference synchronizing signal, which is produced during the operation of the VTR, is supplied to the time base corrector to effect such synchronization of the latter.
In a helical scan type VTR, a rotary magnetic head, referred to as the main head, generally is used for both recording and reproducing video signals on helically-wrapped magnetic tape. In general, another rotary head is used to monitor the video signals which are recorded on the magnetic tape, and this monitoring head also may be used in a general reproducing mode of operation. Also, a so-called dynamic tracking rotary head may be provided for the purpose of reproducing the video signals from the record tracks during normal or special effects reproducing modes. The dynamic tracking head is supported on a displaceable member, such as a bi-morph assembly, this member being responsive to drive signals to be displaced in a direction which is generally transverse of the longitudinal direction of the record tracks. Thus, in the event that the dynamic tracking head does not normally coincide precisely with the record track which it scans, control over the displaceable member brings the head into proper coinciding relationship. The dynamic tracking head finds substantial advantage during special effects modes of operation, such as slow motion, fast motion, stop motion, and the like.
The aforementioned additional reproducing heads, that is, the monitor and dynamic tracking heads, generally are spaced-apart from the main head. During the normal rotary motion of these heads, the monitor and dynamic tracking heads may exhibit an angular displacement from the main head on the order of about 120.degree.. That is, the monitor and dynamic tracking heads exhibit a rotary phase displacement of 120.degree. with respect to the main head.
As is conventional, when video signals are recorded on, for example, magnetic tape in skewed tracks thereacross, a longitudinal control track also is recorded, this control track containing control pulses which are derived from the vertical synchronizing signal included in the video signal. During signal reproduction, these control pulses are reproduced and used in a servo system to control the scanning of the rotary head (or heads) across the tape. A head position pulse generator usually is provided to generate a position pulse whenever the head (or heads) rotates into a predetermined position. These head position pulses are compared with the control pulses which are reproduced from the control track to insure that the rotation of the head (or heads) and the movement of the tape are synchronized. The reproduced control pulses, or the head position pulses, or yet other pulses which are derived from these, are used as so-called reference synchronizing pulses for controlling, or synchronizing the operation of the time base corrector.
Such reference synchronizing pulses generally are in phase synchronism with the vertical synchronizing signal which is reproduced by the main reproducing head of the VTR. That is, when the main reproducing head is used to reproduce the video signals from the magnetic tape, time base correction of such reproduced video signals proceeds properly in response to the aforementioned reference synchronizing signal. However, when the monitor or dynamic tracking head is used to reproduce the video signals, the aforementioned phase synchronism between the reference synchronizing signal and the reproduced vertical synchronizing signal no longer obtains. That is, since the monitor or dynamic tracking head exhibits a phase displacement (on the order of about 120.degree.) with respect to the main head, the vertical synchronizing signal which is reproduced by this monitor or dynamic tracking head likewise exhibits a smaller phase displacement with respect to the reference synchronizing signal. Consequently, if this reference synchronizing signal is not phase-adjusted, time base correction of the video signals which are reproduced by the monitor or dynamic tracking head will not be in synchronism. Thus, operation of the time base corrector will not proceed satisfactorily. As a result thereof, the video picture which ultimately is reproduced from these video signals will be degraded.
To avoid the aforenoted difficulty, the reference synchronizing signal is supplied, substantially as-is, to one input of a selecting circuit, and also is supplied through a delay circuit to another input of this selecting circuit. The delay imparted by the delay circuit corresponds to the angular displacement (e.g. 120.degree.) of the monitor or dynamic tracking head from the main head. When the main head is used to reproduce the video signals from the magnetic tape, the undelayed reference synchronizing signal is selected to be supplied to the time base corrector. When the monitor or dynamic tracking head is used to reproduce the video signals, the delayed version of the reference synchronizing signal is selected. Typically, the selecting circuit is a change-over switch, and a change-over control signal is produced as a function of the particular head which is selected for signal reproduction. This change-over control signal is produced by the VTR and is supplied via a separate cable to the change-over switch, which, typically, is disposed in the time base corrector. However, this additional cable, as well as the separate change-over control signal, may be undesired and, moreover, may result in erroneous operation of the change-over switch due to interference, noise and the like.